Noble gases (also known as rare gases) are an important class of industrial chemicals essential to numerous applications, including lighting, insulation, cryogenic refrigerants, carrier gases, anesthetics, and excimer lasers. Most high-purity, industrial quantities of noble gases are obtained by separating the gas of interest from air.
Currently, most noble gases are obtained via cryogenic distillation—an extremely energy intensive and therefore costly process. Consequently, large-scale isolation of noble gases via selective adsorption near room temperature is highly desirable. Sorption-based processes using porous materials are generally difficult due to the low reactivity of these gases resulting from their filled valence shell. Thus, sorption-based separation processes must rely primarily on differences in atomic size and weak interactions (van der Waals forces) with surfaces. The processes of pressure-swing adsorption (PSA) and vacuum-swing adsorption (VSA) offer attractive alternatives to cryogenic distillation, in that they can be much more energy efficient and therefore cost-effective. However, PSA and VSA require efficient microporous or mesoporous adsorbents that are both highly selective and possess a high capacity. Traditionally, commercial sorbents have been limited to one of four categories: activated carbons, natural and synthetic zeolites, silica gel, and activated alumina. These materials typically have high surface areas and large pore volumes, enabling physisorption of weakly interacting adsorptives such as noble gases. Unfortunately, they also lack synthetic diversity; while currently many examples of commercial sorbents exist, in general, systematic alteration of their structure and chemistry is difficult. Thus, there is a great need for new microporous sorbents with adsorptive properties designed for specific adsorbates.